Abstract Latina adolescents report low levels of physical activity, with only 3% meeting national physical activity guidelines, and are at high lifetime risk for conditions related to inactivity, such as obesity, metabolic syndrome, and diabetes. Interventions grounded in sound psychosocial theory and leveraging growing technology use in Latina adolescents are needed to reverse patterns of inactivity, establish healthy lifetime habits and reduce widening disparities. Our research team has developed and tested individually tailored web-based interventions to increase moderate-to-vigorous physical activity (MVPA), which significantly increased MVPA in Latina adults. In our recent pilot study (R03NR014329), we adapted this intervention based on formative interviews to make it appropriate for Latina adolescents and tested it in a single-arm pilot trial (N=21). After 12 weeks, retention was high (90.5%) and self-reported MVPA increased from 24.7(26.11) minutes/week at baseline to 79.4(46.8) at follow up (p<0.001), suggesting good potential efficacy. In closeout interviews, girls expressed a preference to increase audio-video components of the website and to incorporate other mobile- health technologies, such as texting, smartphone apps, social networks, and wearables. Thus, the aim of this study is to conduct a randomized controlled trial (N=200) of an individually tailored, theory-based, multi- technology intervention to increase MVPA in Latina adolescents. The 12-month intervention will comprise a counseling session to teach behavior change techniques, an interactive multimedia website with individually tailored content, a wearable tracker and smartphone app to reinforce behavior change techniques, connection to an Instagram account reinforcing web content, and automated text messages to guide continued goal setting. Participants will be randomly assigned to receive the intervention or only a wearable tracker. Activity will be measured at baseline, six-months (primary outcome) and 12 months using well-established MVPA measures (accelerometers and the 7-Day Physical Activity Recall Interview). We hypothesize that those randomized to receive the intervention will show significantly greater increases in MVPA at six months than those in the control group. We will also evaluate whether changes in MVPA are mediated by changes in targeted psychosocial constructs (e.g. self-efficacy, social support), and whether intervention effects are moderated by baseline personal and environmental variables (e.g. age, BMI, neighborhood environment). Costs to deliver the intervention (e.g. staff time, materials, overhead) will be tracked to evaluate costs and cost- effectiveness of both study arms, and we will evaluate the effects of contact time out outcomes. We will also explore trajectories of daily activity in both the intervention and control groups using continuous data from wearable trackers. The proposed study will promote a critical preventive health behavior, physical activity, in Latina adolescents using technologies that are pervasive in this high risk population and highly scalable, laying the groundwork for cost-effective, broad reaching interventions with great potential for public health impact.